Radiation-sensitive semiconductor target for a camera tube

ABSTRACT

A radiation-sensitive semiconductor device particularly suitable as a photoconductive target for a camera tube sensitive to long wavelength radiation is constituted by a first layer of semiconductor material having a relatively small band gap, i.e., less than 1.5 eV and a superimposed layer of a semiconductor material having a relatively large band gap, i.e., greater than 1.5 eV. The first layer upon absorbing the incident radiation, generates charge carriers which are injected into the superimposed layer of larger band gap and hence of greater resistivity. The surface of the second layer may then be scanned by an electron beam in conventional manner to derive an output signal indicative of the radiation image impinging on the first layer.

United States Patent [191 Anthony et al.

[ July 3,1973

[ RADIATION-SENSITIVE SEMICONDUCTOR TARGET FOR A CAMERA TUBE [75]inventors: Michel Berth, Antony; Francois Desvignes, Bourg-la-Reine;Claude Piaget, Yerres, all of France [73] Assignee: U.S. PhilipsCorporation, New York,

[22] Filed: Dec. 7, 1971 [21] Appl. No.: 205,570

[30] Foreign Application Priority Data Dec. 10, 1970 France 7044472 [52]U.S. Cl... 317/235 R, 317/235 N, 317/235 NA, 317/235 AC, 317/235 AD [51]Int. Cl. H011 15/00 [58] Field of Search 317/235 N, 235 NA, 317/235 AC,235 AD [56] References Cited UNITED STATES PATENTS 3,408,521 10/1968Dore et a1 313/94 Primary Examiner-Martin H. Edlow Attorney-Frank R.Trilari [57] ABSTRACT A radiation-sensitive semiconductor deviceparticularly suitable as a photoconductive target for a camera tubesensitive to long wavelength radiation is constituted by a first layerof semiconductor material having a relatively small band gap, i.e., lessthan 1.5 eV and a superimposed layer of a semiconductor material havinga relatively large band gap, i.e., greater than 1.5 eV. The first layerupon absorbing the incident radiation, generates charge carriers whichare injected into the superimposed layer of larger band gap and hence ofgreater resistivity. The surface of the second layer may then be scannedby an electron beam in conventional manner to derive an output signalindicative of the radiation image impinging on the first layer.

4 Claims, 2 Drawing Figures Patented July 3, 1973 3,743,899

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Ec1 Eh RADIATION-SENSITIVE SEMICONDUCTOR TARGET FOR A CAMERA TUBE Theinvention relates to a radiation-sensitive semiconductor device forconverting radiation into electric signals, in particular asemiconductor photoconductive target for a camera tube, adapted toproduce an output signal representative of the radiation image impingingon the target.

As is known, the photoconductivity in a semiconductor layer is used incertain television camera tubes, on which layer the scene to be observedis projected by means of a suitable optical system and an output signalis derived from the camera tube representing the image projected on thetarget thereof. In its use as a photo responsive target, thesemiconductor layer is periodically charged by scanning it with anelectric beam of a small cross-section, the charge current varying frompoint to point and being greater at the points of the target which areilluminated the more strongly. Actually, the surface elements of thetarget behave as a plurality of capacitors the leakage currents of whichdepend upon the properties of the semiconductor such as the resistivityand the gradient of the doping concentration, and on properties of theincident radiation, such as the energy and the intensity; the respectivecapacitors discharge more rapidly when the intensity of the radiationimpinging thereon is greater, provided that the energy of the impingingradiation is greater than a threshold which corresponds to the width ofthe band gap of the semiconductor material. The target may consisteither of a thin layer of a semiconductive material having a highresistivity, or of a mosaic of semiconductor junctions which are biassedin the reverse direction by the electron beam. See, for example, U. S.Pat. No. 2,890,359 for the former construction, and U. S. Pat. No.3,579,012 for the latter.

In one known form, the photoconductive semiconductor target consists ofa photoconductive macroscopic homogeneous target of PhD which isobtained by vapor-depositing PbO in a residual atmosphere of oxygen andwater on the inner surface of the entrance window 'of the tube on whicha layer of conductive SnO is previously provided. The target operates atroom temperature, and is sensitive to radiation having a wavelengthsmaller than 0.6 a, which value corresponds to the band gap of PbO whichis approximately 2 eV. The resistivity of the target material is veryhigh (in the order of Ohm.cm) because of the large band gap of thematerial and because of the microscopic discontinuous structure of thelayer. As a result of this, very low dark current values can be obtainedwhich are necessary for the operation of the camera tube. The currentdensity of the dark current preferably is smaller than 10* A cm.

' Similar properties are also found for targets which are manufacturedfrom other semiconductor materials, for example, Se, Sb S or CdS CdSe.

In order to make targets which are sensitive to radiation of longerwavelengths, for example, in the region of the infrared, it is necessaryto use semiconductor materials having a smaller band gap. However, theuse of such target materials is generally not feasible because theresistivity of the material which decreases rapidly with the width ofthe band gap, becomes too low and thereby the dark current of the'cameratube becomes too large.

It is also possible to use the charge storage properties of junctionsfor generating the target signal. In that case the target consists of amosaic of p-n junctions, the capacitance of which may be adapted toproduce a discharge time constant as demanded by the scanning frequencyof the electron beam and at the same time a sufficiently small lateralconductivity is obtained to .produce a readily defined picture. However,even with this expedient the dark current of a junction is neverthelessdetermined by the width of the band gap of the material and the abovenoted disadvantage of homogeneous layers is not avoided.

In view of the foregoing, it has been proposed (see British Pat. No.942,406) to make a camera tube with a target in the form of a mosaic ofgermanium junctions and to operate the target at a temperature which isat most equal to that of solid carbon dioxide at which temperature thespectral sensitivity of the target lies at approximately l.7 n. Thespectral sensitivity may be further shifted by using semiconductorshaving smaller band spacing, for example, lnAs or lnSb which, however,must be cooled to at least 77 K.

The radiation-sensitive semiconductor device which forms the subjectmatter of the present invention has a spectral response at the longerwavelengths and is substantially free from the drawbacks of the priordevices.

According to the invention, a radiation-sensitive semiconductor devicecomprises two semiconductor regions of different materials forming aheterojunction, a first of the said regions serving for absorption ofincident radiation while generating charge carriers, and the secondserving for accumulating or collecting the charge carriers generated inthe first region, the band gap of the material of the second regionbeing larger than the band gap of the material of the first region, andthe band gap and doping of the second region being such that injectionof minority charge carriers can occur from the first region via theheterojunction into the second region.

The second region preferably has such a resistivity that the leakagecurrent is lower than approximately 10 A cm.

In a preferred embodiment, the second semiconductor region is providedas a thin layer on the first region, for example by vapor-deposition.

An important embodiment of the device according to the invention ischaracterized in that the first region is a layer which is deposited ona substrate which is substantially permeable to the incident radiation.In this construction the permeable substrate may advantageously form anohmic contact with the first region.

, The semiconductor material of the first region preferably has a bandgap smaller than 1.5 eV.

The semiconductor material of the second region preferably has a bandgap exceeding 1.5 eV.

In order that the invention may be readily carried into effect, it willnow be described in greater detail, by way of example, with reference tothe accompanying drawings.

HO. 1 shows schematically a band diagram corresponding to aradiation-sensitive device according to the invention.

FIG. 2 shows diagrammatically a camera tube comprising aradiation-sensitive device according to the invention.

In accordance with the invention, a semiconductor device particularlysuitable as a target for a camera tube responsive to long wavelenthradiation, comprises superimposed layers or regions of two differentsemiconductor materials forming there between a heterojunction. Thechoice of the material of the first region in which the radiationabsorbtion takes place is related to the wavelenth of the incidentradiation. For example, InAs is suitable for detection of radiation to awavelength of 3.5 p. and lnSb to a wavelength of 5.6 t. The chargeaccumulation or storage function cannot be fulfilled by this samesemiconductor unless it is cooled to a very low temperature, because ofthe small band gap of the material of the first region. According to theinvention, for the accumulation or storage function anothersemiconductor material having a larger band gap is used, for example,germanium, silicon or composite materials with, for example, a commonconstituent with the absorbing semiconductor of the first region, suchas lnP, AlSb, GaAs. This second semiconductor material constitutes thesemiconductor material of the second region.

It is to be noted that for materials with a band gap smaller than 1.5eV,preferably used for the first region, the resistivity of the layer islower than is desirable. In such instances the resistivity may beincreased either by cooling or by creating a disturbed or amorphouscrystal structure over at least a part of the thickness of the layer.This crystal structure is obtained either by deposition in a vacuumunder suitable conditions at a suitable substrate temperature or by asuitable rate of deposition. The resistivity may also be increased byion bombardment of the deposited layer.

Consider, for example, the detection of a radiation of wavelength A 5 a.A semiconductor material suitable for detecting such radiation is lnSb.The device may be manufactured, for example, as follows:

On substrate semiconductor substRate of, for example, n-type germanium,a layer of lnSb is deposited according to the so-called threetemperatures method (see for example, US. Pat. No. 3,441,000) with athickness which may vary to, for example, 20 pm. The deposited layer isof the n-type and has a donor concentration N D in the low cm range. Onthis layer is vapor-deposited in a high vacuum a layer of substantiallyintrinsic germanium having a resistivity of approximately 50 Ohm. cm anda thickness of a few microns; the resistivity at the surface of saidgermanium layer is then increased by a bombardment with argon ions theenergy of which is approximately 1 keV under a pressure which is reducedto approximately 10 Torr, the semiconductor layer being maintainedapproximately at room temperature. The argon bombardment is continueduntil the germanium resistivity becomes substantially equal toapproximately 10' Ohm.cm (at 77 K). It has been found experimentallythat a later thermal treatment in hydrogen or in a vacuum, as a resultof which the resistivity decreases, can also improve the sensitivity ofthe device.

It is also possible to use, instead of a thin deposited layer of lnSb, aplate of monocrystalline lnSb the effective operating portion of whichis previously made thinner by a careful chemical etching until athickness of 20 to 50 p. is obtained. The remaining processing is thesame.

In the first example given above, an annular thermal and electriccontact can be provided on the germanium substrate; in the secondexample, the contact is formed on the outer ring of the lnSb plate whichhas not been made thinner. Soldering is carried out by alloying withindium in pure hydrogen on a flange of a ferro-nickel alloy having asuitable coefficient of expansion, available commercially as Dilver P,and the flange is then secured to a cooling element.

Such a target can operate only at low temperatures owing to the presenceof lnSb, a semiconductor having a very small width of the forbiddenband.

FIG. 1 shows diagrammatically a band gap diagram which corresponds tosuch a target. The negative polarity of the surface of the second region(II) as the result of the scanning electron beam creates a potentialgradient which is present mainly in the germanium (ll) of highresistivity. When the free carriers generated in the lnSb first region(I) in which the radiation absorbtion takes place, are injected with anefficiency which is not equal to zero via the interface into the secondregion (II) either by thermal effect or field effect, or by tunneleffect from band to band or from band to recombination center a changein the potential difference occurs at the region of the targetcorresponding to the impingement point of the incident radiation betweenthe surfaces of the layer (II) and consequently a variation of theleakage current at this point of the target.

In this manner a charge pattern is produced in the target by means of asingle junction, the heterojunction, which pattern corresponds to theintensity distribution of the radiation incident on the target, whichcharge pattern can be read by means of one of the conventional scanningmethods.

The camera tube (FIG. 2) comprises within an envelope 8 which isprovided with a window 1 which is transparent to the incident radiation,a transparent support 2 of approximately 25 mm diameter on which thelayers 3 of lnSb and Ge are deposited, said support being surrounded bya cooling ring provided with an inlet for a cryogenic liquid and onwhich a diaphragm 5 is secured which screens the radiation from thesurroundings on the rear side.

According to another embodiment, a layer of CdTe is deposited on thelnSb.

In these various embodiments, the cooling, the object of which is toreduce the number of free carriers in the lnSb, is carried out at atemperature (liquid nitrogen 77 K) which is considerably higher than thetemperature which would be necessary if this semiconductor material wereto be used to accumulate such carriers by means of a p-n junction(liquid helium 4 K).

A usual electronic optical system 7 enables the scanning and focusing ofthe electron beam originating from a gun 6.

According to another embodiment, the target may consist of a circularplate having a diameter of approximately 25 mm and consisting of lnSb onwhich germanium is deposited, the plate being mounted on the coolingring in a glass assembly as used in the preceding example.

Referring back to FIG. 1, E,, and E represent the valence band levelsfor the band diagrams of the regions l and ll, E and E represent thecorresponding conduction band levels, and E and E the correspondingFermi levels. A E is the band gap for region I and A E is the band gapfor region II. The lower arrows designated l1, l2 and 13 symbolizeinjection of minority carriers from region I into region II, and theupper arrows designated 14 and 15 symbolize flow of electrons.-

We claim:

l. A camera tube comprising a photo-conductive target plate, means forimpinging an incident radiation image on one side of the target plate,means for electrically contacting said one side of the target plate,means for scanning the opposite side of the target place with anelectron beam, said target plate comprising a first layer adjacent theone side and a second layer adjacent the opposite side and adjoining thefirst layer, the first and second layers being of differentsemiconductive materials forming a heterojunction where they adjoin oneanother, the first layer having a band gap at which the incidentradiation is absorbed therein generating free charge carriers, thesecond layer having a band gap that is larger than the band gap of thefirst layer and having a resistivity that is higher than the resistivity0F the first layer whereby minority charge carriers generated in thefirst layer will be injected into the second layer via theheterojunction.

2. A camera tube as set further in claim 1 wherein the band gap of thefirst layer is smaller than 1.5eV, and the band gap of the second layeris larger than 1.5eV.

3. A camera tube as set further in claim 2 wherein the second layer is agermanium or silicon. v

4. A camera tube as set forth in claim 2 wherein the first layer ismonocrystalline.

2. A camera tube as set further in claim 1 wherein the band gap of thefirst layer is smaller than 1.5eV, and the band gap of the second layeris larger than 1.5eV.
 3. A camera tube as set further in claim 2 whereinthe second layer is a germanium or silicon.
 4. A camera tube as setforth in claim 2 wherein the first layer is monocrystalline.